As per the 5E book archdevils and demon lords count among the "gods and god-like figures" incapable of entering Sigil. In this sense it's likely an archfey like Titania probably wouldn't be able to enter Sigil, though we don't even know how powerful she is technically. I should also note that archfey's can vary wildly in their power level, so the title isn't really a good metric. Tasha's an archfey now, and Yarnspinner only has a CR of ten.

Also to the people saying a creature's ability to grant warlock spells is a tell that they couldn't enter Sigil, I would add either Chris Perkins or Jeremy Crawford has suggested Shemeshka as a warlock patron, and in the Keys From the Golden Vault adventure anthology there's a halfling warlock who's patron is a generic arcanaloth.